This invention relates to an improved process for the production of detersive granules and to a detergent which consists predominantly of the conventionally formulated granules thus produced.
Although the trend today is increasingly towards heavy detergents with bulk densities of 650 g/l and higher, preferably above 700 g/l, there is still a demand for detergents with bulk densities below 700 g/l. Above all in parts of the world where handwashing still plays an important part or where, for example, tub-type washing machines are still predominantly used for machine washing, the detergents used are expected to dissolve quickly without excessive mechanical assistance. In the case of mixed and granulated products, this has hitherto been achieved through the reduced bulk density of the detergents.
Modern detersive granules are normally expected to show adequate stability in storage in regard to the flowability of the granular products. Today, this requirement is normally satisfied with the optional assistance of so-called surface modifiers which cover the surface of the granules and prevent them from sticking to one another. However, another requirement, namely producing a macroscopically homogeneous product, which does not undergo any separation during production or packaging and which does not show any separation of specifically different powder qualities, by mixing and granulation in order to prevent separation of the individual components during transportation or storage, can still present the expert with problems depending on the raw materials used and the apparatus available. If the need for a certain bulk density coupled with high formulation variability is then added as yet another requirement, the expert is obliged to make compromises among the various possibilities hitherto available.
Although relatively free-flowing and homogeneous products are obtained in conventional spray drying processes, hydrolysis-sensitive or temperature-sensitive detergent ingredients, for example peroxy bleaching agents or enzymes, have to be subsequently incorporated. Since the direct spray drying product normally has bulk densities of only 300 to 550 g/l, it has to be converted into granular form if higher bulk densities are required, as sufficiently well-known from the patent literature. If heavy ingredients are merely incorporated by mixing, an increase in bulk density is certainly obtained, but only at the expense of a risk of separation during transportation and storage. In addition, spray drying is a cost-intensive process, so that it is economically unfavorable to produce the principal component of a detergent by spray drying.
In addition, it is generally known that the content of anionic surfactants in particular in spray-dried granules has to be limited both for production and safety reasons (risk of fire) and for applicational reasons (lump formation). For example, spray-dried granules containing more than 20 to 25% by weight of alkyl benzenesulfonates have a marked tendency to form lumps. Accordingly, highly concentrated granules containing anionic surfactants cannot be produced by spray drying.
Various mixers and granulators in which either heavy or relatively light granules can be produced are available today. Thus, a high bulk density is obtained, for example, in a Lodige plowshare mixer (in approximate terms, the bulk density predicted by the normal method of calculation "sum of the percentages by weight of the individual solid raw materials multiplied by their bulk densities and the liquid components multiplied by their density" is obtained or the bulk densities are only slightly lower than that value if the mixer is efficiently operated), although agglomeration is normally inadequate so that inhomogeneous granules and a relatively broad particle size range with coarse and fine particle components are obtained. In addition, the relatively coarse-particle solids used undergo at least partial destruction. These products tend to separate.
Whereas mixers and granulators, such as the plowshare mixer, can be characterized by rotating tools, the so-called free-fall mixers are distinguished by the fact that they do not contain any tools and belong to the mixers with rotating containers. In free-fall mixers, the product being mixed is lifted by friction with the wall or internals and "trickles" back down onto the surface of the pile under the effect of gravity.
In so-called double-cone mixers, which belong to the free-fall mixers and in which detersive granules with bulk densities that confirm the theoretical calculation are normally obtained, the solid ingredients are gently mixed without particle destruction in contrast to the plowshare mixer. Unfortunately, the product remains inhomogeneous which is an indication of inadequate agglomeration.
Apart from a few exceptions where paste-form starting materials are granulated, one or more solids is/are normally processed with the assistance of granulation liquids in the process of mixing and agglomeration. Thus, International patent application WO-A-97/21487, for example, describes a process for the production of detersive granules in which water or aqueous solutions and/or aqueous dispersions are only added in such quantities that the water binding capacity of the final stable granules is not exceeded. The bulk densities of the embodiments mentioned in the Examples are between 650 g/l and 780 g/l. There is no reference to the homogeneity of the product or to an optionally adjustable bulk density of the granules. However, the preferred choice of mixers/granulators which accommodate a high energy input suggests that, for a given formulation, the bulk densities are not freely adjustable and/or the final granules show distinct inhomogeneities in accordance with the foregoing observations.
Another problem lies in the homogeneous incorporation of minor components which are only used in small quantities, for example in quantities of up to about 10% by weight, in a detergent. These minor components include co-builders, optical brighteners, sequestering agents, redeposition inhibitors, soap, dyes and perfumes, etc. German patent application DE-A-196 51 072 suggests accommodating minor components of the type in question in a separate additive, the use of this additive providing for more exact dosing and for more homogeneous distribution of the minor components throughout the detergent.
The two above-cited documents alone show that mixed products normally contain basic granules to which several other components are subsequently added or that several compounds (each containing at least two detersive ingredients) are separately produced and subsequently mixed, optionally with incorporation of other raw materials. Typical added components are, for example, peroxy bleaching agents, such as perborate and/or percarbonate, which can have bulk densities of 800 to 1000 g/l, or sodium sulfate which has a bulk density of up to 1500 g/l and which may still be present in quantities of up to 45% by weight in some detergents. Even optionally heavy sodium carbonates or bleach activators are suitable as added components. The added components mentioned with bulk densities above 700 g/l can be incorporated relatively easily in heavy detergents. However, in detergents expected to have bulk densities below 650 g/l, not only must the other components have a correspondingly lower bulk density, there is also a serious risk of separation due to the differences in bulk density between the individual granular components. In the case of heavy sodium sulfate, there is the further complication that sodium sulfate consists of relatively fine particles and tends in any case to sink to the bottom of detergent packs during storage and above all during transportation.
Tetraacetyl ethylenediamine (TAED), which is still the most commonly used bleach activator, has bulk densities of only 500 to 600 g/l. However, in detergents which have a bulk density of only 400 g/l, even TAED is regarded as a heavy and hence difficult-to-handle raw material.
Now, the problem addressed by the present invention was to enable detersive granules which, besides showing adequate stability in storage (flowability), would above all be homogeneous to be produced by mixing and agglomeration. Above all heavy ingredients with bulk densities above the required bulk density of the end product would lend themselves to processing without the end products showing any tendency to separate.